This is a very common practice to not list people who don't work at a company when the credits are made. It is also common to list people who didn't work on the game, as long as they work at the company when the credits are made.
To those who are saying it's incredibly terrible and that you wouldn't buy a product done that way...keep in mind...many of the companies you love actually do this. I've personally never understood why (i have to assume there is some weird legal reason), but it's common.
I've never been effected by it or known anyone effected by it because probably 90% or more of hires (of significant value) in the game industry result from a process that starts from word of mouth and/or networking.